1. Field of the Invention
The present invention relates to light-receiving and amplifying devices, and more particularly to a light-receiving and amplifying device having a photodiode and an amplifier circuit to be used as a constituent part of a pickup for a magneto-optic disc player, a mini-disc player, or the like.
2. Description of the Prior Art
Referring to FIG. 7, a conventional light-receiving and amplifying device of the above type is provided with four photodiodes PDA, PDB, PDC, and PDD having a common cathode and four amplifier circuits AMPA, AMPB, AMPC, and AMPD which receive photocurrent signals I.sub.PDA, I.sub.PDB, I.sub.PDC, and I.sub.PDD from the anodes of the photodiodes PDA, PDB, PDC, and PDD. PDE indicates a parasitic photodiode. The amplifier circuits AMPA, AMPB, AMPC, and AMPD generate low-impedance voltage outputs V.sub.A, V.sub.B, V.sub.C, and V.sub.D. A pickup carries out signal processing operations such as a calculation operation based on the voltage outputs V.sub.A, V.sub.B, V.sub.C, and V.sub.D to regenerate the data recorded on a disk (recording medium).
The above-mentioned photodiodes PDA, PDB, PDC, and PDD are fabricated in a N-type epitaxial layer 12 provided on a P-type semiconductor substrate 10 as shown in FIGS. 8 and 9 (where FIG. 8 shows a pattern layout, while FIG. 9 shows a cross-sectional construction). In more detail, four P-type diffusion layers 15, 16, 17, and 18 are formed on a surface of the N-type epitaxial layer 12, where the P-type diffusion layers 15, 16, 17, and 18 and the N-type epitaxial layer 12 constitutes the photodiodes PDA, PDB, PDC, and PDD. The above-mentioned N-type epitaxial layer 12 is separated from its periphery by means of a P-type isolation diffusion layer 13. It is noted that the reference numeral 11 denotes a N-type buried layer, and the reference numeral 14 denotes a N-type collector compensating diffusion layer. In the above-mentioned construction, there is the parasitic photodiode PDE between the common cathode of the photodiodes PDA, PDB, PDC, and PDD and the P-type semiconductor substrate 10 as shown in FIG. 9.
There are contained many DC light components in the input signal (representing light reflected on the disk) to the pickup. In the aforementioned conventional light-receiving and amplifying device, the photocurrent signals I.sub.PDA, I.sub.PDB, I.sub.PDC, and I.sub.PDD containing the DC components are directly input to the amplifier circuits AMPA, AMPB, AMPC, and AMPD. The above-mentioned fact leads to a problem that the device has a narrow operation range of AC components. Particularly a pickup for a portable appliance is required to operate on a low voltage of 3 V, and therefore the AC operation range is further narrowed.
As a means for solving the above-mentioned problem, there can be considered a method of removing the DC components with provision of a high-pass filter circuit between the photodiodes PDA, PDB, PDC, and PDD and the amplifier circuits AMPA, AMPB, AMPC, and AMPD to let only the AC components pass. However, a pickup for use in an audio appliance is required to operate at a relatively low disk address signal frequency of about 22 kHz. Therefore, the above-mentioned high-pass filter circuit necessitates a capacitor having a large capacitance, which results in the dimensional increase of the device. Furthermore, when the photodiodes PDA, PDB, PDC, and PDD and the amplifier circuits AMPA, AMPB, AMPC, and AMPD are integrated in an identical chip, a complicated circuit results to increase the production cost.